Posting this just because I spent the time to look it up and it is a pain to find.
The pertinent law is District of Columbia Official Code, Division I, Title 7, Subtitle J, Chapter 25, Unit A, Subchapter VI, § 7-2506.01:
(b) No person in the District shall possess, sell, or transfer any large capacity ammunition feeding device regardless of whether the device is attached to a firearm. For the purposes of this subsection, the term “large capacity ammunition feeding device” means a magazine, belt, drum, feed strip, or similar device that has a capacity of, or that can be readily restored or converted to accept, more than 10 rounds of ammunition. The term “large capacity ammunition feeding device” shall not include an attached tubular device designed to accept, and capable of operating only with, .22 caliber rimfire ammunition.
The penalty (if I understand how the law is laid out correctly, I am not a lawyer), is in District of Columbia Official Code, Division I, Title 7, Subtitle J, Chapter 25, Unit A, Subchapter VII, § 7-2507.06:
Any person convicted of a violation of any provision of this unit shall be fined not more than $1,000 or imprisoned for not more than 1 year, or both…
The full text is found here.